diff --git a/src/acs/environment.cpp b/src/acs/environment.cpp
index 48a16fdc8..4017752b5 100644
--- a/src/acs/environment.cpp
+++ b/src/acs/environment.cpp
@@ -350,10 +350,13 @@ ACSVM::Word Environment::callSpecImpl
 
 	activator_t *activator = static_cast<activator_t *>(Z_Calloc(sizeof(activator_t), PU_LEVEL, nullptr));
 	auto __ = srb2::finally(
-		[activator]()
+		[info, activator]()
 		{
-			P_SetTarget(&activator->mo, NULL);
-			Z_Free(activator);
+			if (info->thread_era == thinker_era)
+			{
+				P_SetTarget(&activator->mo, NULL);
+				Z_Free(activator);
+			}
 		}
 	);
 
diff --git a/src/acs/thread.hpp b/src/acs/thread.hpp
index 0b20271af..f72aac34f 100644
--- a/src/acs/thread.hpp
+++ b/src/acs/thread.hpp
@@ -59,6 +59,7 @@ enum acs_tagType_e
 class ThreadInfo : public ACSVM::ThreadInfo
 {
 public:
+	UINT32 thread_era;			// If equal to thinker_era, mobj pointers are safe.
 	mobj_t *mo;					// Object that activated this thread.
 	line_t *line;				// Linedef that activated this thread.
 	UINT8 side;					// Front / back side of said linedef.
@@ -67,6 +68,7 @@ public:
 	bool fromLineSpecial;		// Called from P_ProcessLineSpecial.
 
 	ThreadInfo() :
+		thread_era { thinker_era },
 		mo{ nullptr },
 		line{ nullptr },
 		side{ 0 },
@@ -77,6 +79,7 @@ public:
 	}
 
 	ThreadInfo(const ThreadInfo &info) :
+		thread_era { thinker_era },
 		mo{ nullptr },
 		line{ info.line },
 		side{ info.side },
@@ -88,6 +91,7 @@ public:
 	}
 
 	ThreadInfo(const activator_t *activator) :
+		thread_era { thinker_era },
 		mo{ nullptr },
 		line{ activator->line },
 		side{ activator->side },
@@ -100,11 +104,15 @@ public:
 
 	~ThreadInfo()
 	{
-		P_SetTarget(&mo, nullptr);
+		if (thread_era == thinker_era)
+		{
+			P_SetTarget(&mo, nullptr);
+		}
 	}
 
 	ThreadInfo &operator = (const ThreadInfo &info)
 	{
+		thread_era = thinker_era;
 		P_SetTarget(&mo, info.mo);
 		line = info.line;
 		side = info.side;
diff --git a/src/p_local.h b/src/p_local.h
index fccc0287f..1e5eee012 100644
--- a/src/p_local.h
+++ b/src/p_local.h
@@ -79,6 +79,7 @@ typedef enum
 extern thinker_t thlist[];
 
 void P_InitThinkers(void);
+void P_InvalidateThinkersWithoutInit(void);
 void P_AddThinker(const thinklistnum_t n, thinker_t *thinker);
 void P_RemoveThinker(thinker_t *thinker);
 void P_UnlinkThinker(thinker_t *thinker);
diff --git a/src/p_tick.c b/src/p_tick.c
index 1ad1c8dd2..98ca4b690 100644
--- a/src/p_tick.c
+++ b/src/p_tick.c
@@ -53,6 +53,8 @@
 tic_t leveltime;
 boolean thinkersCompleted;
 
+UINT32 thinker_era = 0;
+
 static boolean g_freezeCheat;
 static boolean g_freezeLevel;
 
@@ -276,6 +278,8 @@ void P_InitThinkers(void)
 {
 	UINT8 i;
 
+	P_InvalidateThinkersWithoutInit();
+
 	for (i = 0; i < NUM_THINKERLISTS; i++)
 	{
 		thlist[i].prev = thlist[i].next = &thlist[i];
@@ -297,6 +301,15 @@ void P_InitThinkers(void)
 	Obj_ResetCheckpoints();
 }
 
+//
+// P_InvalidateThinkersWithoutInit
+//
+
+void P_InvalidateThinkersWithoutInit(void)
+{
+	thinker_era++;
+}
+
 // Adds a new thinker at the end of the list.
 void P_AddThinker(const thinklistnum_t n, thinker_t *thinker)
 {
diff --git a/src/p_tick.h b/src/p_tick.h
index 58c154a1a..ab36c2e51 100644
--- a/src/p_tick.h
+++ b/src/p_tick.h
@@ -43,6 +43,8 @@ void P_PreTicker(INT32 frames);
 void P_DoTeamscrambling(void);
 void P_RemoveThinkerDelayed(thinker_t *thinker); //killed
 
+extern UINT32 thinker_era;
+
 mobj_t *P_SetTarget2(mobj_t **mo, mobj_t *target
 #ifdef PARANOIA
 		, const char *source_file, int source_line